In the development of vehicles such as an automobile, in order to cope with the problems such as weight reduction, development period reduction and experimental vehicle manufacture reduction, in recent years, prediction of each performance based on numerical analysis with the use of a computer is frequently performed in designing field.
For instance, as for a crash absorbed energy, in order to absorb crash energy arising when the automobile crashes, a member such as a front side member is designed to cause a regular buckling in the longitudinal direction thereof at the time of the crash to absorb impact energy by plastic deformation by the buckling, so that an occupant of the automobile is protected.
In conventional designing of a crash energy absorption member, after an initial shape of the member is determined, crash analysis is performed such as by finite element method, and a change in shape and so forth is made to the member so that the crash absorbed energy attains a target value. After the evaluation by the analysis has attained the target value, a final confirmation is made by experimental manufacture and experiment, so that the design is determined.
These members are manufactured by performing plastic working to sheets, tubes or bars made of steel or other material and, as appropriate, by joining them. For the plastic working, a forming method such as pressing, hydroforming, or extrusion or the like is adopted. Also, for the joining, a method such as spot welding, arc welding, laser welding, or rivet connection or the like is adopted.
Conventionally, an approach referred to as coupled analysis reaching from press forming to crash analysis shown in FIG. 25 is known, and, in Japanese Patent Application Laid-Open No. 2004-50253 (Patent document 1), there is disclosed a simulation technique, in which, based on the final shape data of a pressed part, an additional shape data is prepared and forming analysis is performed thereafter, and characteristic analysis such as on an ability to withstand a crash force is performed based on the obtained analysis result in a coupled manner. In FIG. 25, “2501” denotes an unprocessed material, “2502” denotes the result of the forming analysis, “2503” denotes the result of the forming analysis after converted into input data for the crash analysis, and “2504” denotes the result of the crash analysis.
However, in Patent document 1, no description is given as to an approach to present an optimum part shape and a forming condition.
It is known that, when a metal such as a steel material is used as a material, a variation in sheet thickness is caused due to plastic working when manufacturing the member and/or work hardening is caused due to the plastic strain, in which the buckling deformation mode and/or crash absorbed energy vari(es) when the member suffers the crash as compared to the case where no sheet thickness variation or working hardening is caused.
Under current condition, since neither sheet thickness variation nor work hardening is taken into consideration at the time of the analysis such as by the finite element method or the like, even if the designing is performed based on the evaluation value obtained by the analysis, desired buckling deformation mode and/or crash absorbed energy cannot be obtained in the experimental manufacture or experiment.
Further, due to a fluctuation in a plastic working condition when manufacturing the member, there arise(s) fluctuation(s) also in sheet thickness variation and/or work hardening, finally causing the fluctuation (s) in the buckling deformation mode and/or crash absorbed energy.
Further, the buckling deformation mode and the crash absorbed energy vary depending on a butt weld line position when using a tailored blank and a joining condition when joining a plurality of members.
Also, as for fatigue strength evaluation for a vehicle, the development needs for an approach allowing accurate and easy prediction of the fatigue life of a part, a member or a structure used for the vehicle aiming limit design are increasing more than ever.
In this field, conventionally, static stress analysis under the condition of a predetermined fatigue load by the finite element method is widely used, and when predicting the fatigue life using the analysis result, an approach; in which an initial shape is determined first and fatigue test data (S-N diagram, E-N diagram) of materials previously used for the member and of the joining portion are obtained, predictive calculation is performed at the same time by cross checking the diagrams, a stress analysis value or a strain analysis value to obtain a predictive life, and then a change is made to the shape of the member, the material, the joining method, or the like so that the calculated fatigue life becomes the target value; is adopted.
After the evaluation by the analysis attains the target value, verification is performed by experimental manufacture and experiment to determine a design specification. These members are manufactured by performing plastic working to sheets, tubes or bars made of steel or other material and, as appropriate, by joining them. For the plastic working, a forming method such as pressing, hydroforming, or extrusion or the like is adopted. Also, for the joining, a method such as spot welding, arc welding, laser welding, or rivet connection or the like is adopted. Recently, fatigue analysis software automatically, which refers to a stress calculation result file obtained by the finite element method as well as fatigue test data of the material previously used for the member and of the joining portion and thereby calculates the life of respective portions, is commercially available.
When a metal such as steel is used as a material, due to the plastic working when forming the member, the sheet thickness variation and plastic strain are caused, and at the same time, when the member is assembled, residual stress due mainly to springback after the formation of the member is caused, and those sheet thickness variation, plastic strain and residual stress are known to largely affect the member in fatigue strength. Further, the calculation method of the residual stress when assembling the member and the quantification method of the fatigue strength variation due to the plastic strain are not clearly defined, making it difficult to build an optimization algorithm for fatigue design of a member to obtain a forming work method satisfying a targeted fatigue life. In the conventional method, effects when forming and assembling those are not taken into consideration, and, on top of that, no optimization algorithm is adopted, so that the fatigue design of the member cannot be performed accurately and speedy, as a matter of fact.
In Japanese Patent Application Laid-Open No. 2001-116664 (Patent document 2), in the analysis method analyzing fatigue strength of a weldment structure composed of plural members, there is disclosed a fatigue strength analysis method evaluating the fatigue strength, in which, based on the shapes and welding methods of the two welded members, in view of a weld line portion, a fatigue strength diagram in the parallel direction to the weld line and a fatigue strength diagram in the vertical direction to the weld line are selected, respectively, and with the stress analysis result of this weldment structure, the stresses in the vertical and parallel directions to the weld line are obtained, and then by comparing these stresses with the fatigue strength diagrams, respectively, to evaluate the fatigue strength.
However, in the method disclosed in Patent document 2, the residual stress arising at each portion after assembling, the plastic strain given when forming the member, and the post-formation sheet thickness distribution are not taken into consideration, and that, the optimization algorithm is not adopted, leaving a problem that the fatigue life prediction cannot be performed accurately and speedy.
In Japanese Patent Application Laid-Open No. 2003-149091 (Patent document 3), a fatigue life evaluation system is disclosed, in which a stress concentration ratio corresponding to a welded shape (finishing process) of a welded portion is recognized beforehand for each joint type by experiment or the like to be stored in a memory together with fatigue life prediction data (S-N diagram) of a front structure, stress of the welded portion is calculated by finite element method analysis, peak stress at an end of the portion welded by bead is calculated by multiplying the stress value by the stress concentration ratio corresponding to the welded shape, and the peak stress is applied to the S-N diagram to predict the fatigue life in accordance with the welded shape.
In Japanese Patent Application Laid-Open No. 2003-149130 (Patent document 4), there is disclosed a method, in which a shell model for a finite element method analysis is prepared with respect to a spot-welded structure composed of sheets fitted together; and linear and elastic analysis by the finite element method is performed using the prepared shell model for the finite element method analysis to calculate a shared load at a nugget portion at a center of the spot welded portion as well as a deflection on and a radial tilt angle of a circle drawn around the nugget portion and having a diameter of D; and then, based on the calculated shared load and the deflection on and radial tilt angle of the circle, nominal structural stress at the nugget portion is obtained using the circular plate bending theory of the elasticity theory to predict the fatigue life of the spot-welded structure using the nominal structural stress.
However, in the methods disclosed in Patent documents 3 and 4, the residual stress arising at each portion after the assembling, the plastic strain given when the member is formed, and post-formation sheet thickness distribution are not taken into consideration, and that, the optimization algorithm is not adopted, leaving a problem that the fatigue design cannot be performed accurately and speedy.